1. Field of the Invention
This invention is an electromagnetic wave absorbing sheet, which can effectively absorb electromagnetic waves in a frequency range over 1 GHz.
2. Description of the Prior Art
With the recent advanced technology, various electromagnetic machines are widely used. There is a possibility that electromagnetic waves radiated from these electromagnetic machines have an influence on a control and performance of the electronic machines and a daily life.
Various devices have been proposed such as mounting a shield around a radiator of electromagnetic waves or other devices have been provided at the electronic machines to prevent leakage of electromagnetic waves.
In addition, various techniques have been proposed to reflect or receive the radiated electromagnetic waves for the purpose of avoiding the influence of the emitted electromagnetic waves.
Normally, an electromagnetic wave receiver which consists mainly of magnetic material such as ferrite is used near a frequency range between 10 MHZ-1 GHz, and conductive material such as carbon, magnetic material such as carbonyl iron, metallic fiber, conductive fiber and conductive paint are used in a frequency range over 1 GHz.
A nonwoven fabric sheet comprising metallic fibers for absorbing electromagnetic waves has been proposed, for example, Japanese Patent Publication Nos. 12898/1990 and 307268/1997.
The conductive high polymer fibers are metallic fibers, each fiber having a diameter of 20 xcexcmm and a length of 5 mm, which is described in Japanese Patent Publication No. 12898/1990.
The metallic fibers, each fiber having a diameter of 10-100 xcexcm, a length of 1-5 mm and being penetrated into a pressure-molded resin, are described in Japanese Patent Publication No. 188190/1993.
The metallic fibers, each fiber having a length more than 1 mm and a thickness less than {fraction (1/10)} of the length, are described in Japanese Patent Publication No. 288684/1996.
In view of the fact that the fibers of the nonwoven fabric sheets for absorbing electromagnetic waves are relatively linear, not easily got caught with each other, hardly cut off in a manufacturing process and can be arranged along the nonwoven fabric sheet which is being produced.
The production stage of any absorbing sheet for electromagnetic waves, however, is very complicated so that the cost of the produced absorbing sheet increases.
A wire diameter of each metallic fiber is substantially uniform, and a length thereof is also uniform so that reflecting attenuation effect of incident electromagnetic waves is remarkably different when a plane of polarization of electromagnetic wave is in parallel with a longitudinal direction of the absorbing sheet and when the plane thereof is vertical to the absorbing sheet.
There is another annoying problem that absorption quantity of the electromagnetic wave is different with reference to a relationship between the direction (longitudinal direction) of the absorbing sheet and the plane of polarization thereof, and that the absorbing quantity is large when the plane of polarization is in parallel with the longitudinal direction of the absorbing sheet (hereinafter referred to as heterotrophy).
It should be appreciated that an induction ratio of the conventional electromagnetic wave absorbing sheet is remarkably different when the plane of polarization of the electromagnetic waves is in parallel to the longitudinal direction of the absorbing sheet and when the plane is vertical to the absorbing sheet.
Heterotrophy is described in the paper of xe2x80x9cInternational Symposium on Electromagnetic Compatibility in Nagoyaxe2x80x9d, Sep. 8-10, p. 775, 1989, Tetsuji Inui et al.).
The metallic fiber enclosed in the nonwoven fabric sheet has a diameter of 8 xcexcm, a length of 40 mm and a mass ratio of 0.3-3 wt %, and it is considered that the metallic fiber causes heterotrophy.
Any of the conventional electromagnetic wave sheets has a drawback that a special design has to be worked out in order to obtain a full absorbing performance for both parallel and vertical polarization planes of the electromagnetic wave.
A principal object of this invention is to provide an electromagnetic wave absorbing sheet which, comprises a nonwoven fabric sheet including one or a plurality of nonwoven fiber layers, each layer comprising non-metallic fibers having no binding faculty, non-metallic binding fibers having binding faculty which are melted by heat to bind neighboring fibers, and metallic fibers having an irregular shape in cross-section and being produced by shaving a metallic wire or rod, that are dispersed uniformly in the nonwoven fiber layer or layers; and a metallic layer having electromagnetic wave reflecting faculty and disposed on an outer bottom surface of the nonwoven fiber layer or layers.
Another object of this invention is to provide an electromagnetic wave absorbing sheet, which can be provided at a given radiator of electromagnetic waves.
Still another object of this invention is to provide an electromagnetic wave absorbing sheet, which can be fabricated at a low cost.